1. Field of the Invention
The present invention relates generally to electrical power conversion, and particularly to a supercapacitor-based grid fault ride-through system that enhances grid fault ride-through capability of a wind power generation system.
2. Description of the Related Art
The increased level of wind penetration into the power system has resulted in the revision of Grid Codes for wind generators in many countries. Grid codes issued during the last few years invariably demand that wind farms must withstand voltage dips to a certain percentage of the nominal voltage (down to 0% in some cases) and for a specified duration. Such requirements are known as Fault Ride Through (FRT) or Low Voltage Ride Through (LVRT), and are described by a voltage vs. time characteristic curve.
The FRT requirements also include fast active and reactive power restoration to the pre-fault values after the system voltage returns to normal operation levels. Some codes impose increased reactive power generation by the wind turbines during the disturbance in order to provide voltage support. Nevertheless, the wind parks are required to behave, in principle, like other generators, e.g., conventional generating units.
Induction generators (IGs) are widely used as wind generators since they are relatively inexpensive, rigid, and require low maintenance. Although variable-speed wind turbines (WTs) are getting popularity, statistics show that a significant percentage of the total installations are of fixed-speed WTs with cage-rotor IG. But this kind of generator consumes a large amount of reactive power during normal operating conditions. This consumption jumps sharply upon the occurrence of grid fault due to a large increase in slip. Instability in induction generator terminal voltage emerges from the deficiencies in the supply of demanded reactive power during the transient period.
Solutions that have been suggested to prevent voltage collapse after the faults include dynamic reactive power sources, i.e., a static reactive volt-ampere compensator (SVC) or a static synchronous compensator (STATCOM). But a common STATCOM does not have the capability of active power compensation because the DC capacitor with it is not a bulk storage device. It can only affect the active power flow in the power system indirectly by regulating the voltage at the common point of connection with the transmission line. A STATCOM with a braking resistor or a battery energy storage system (BESS) have also been proposed for the stability enhancement of a large wind farm. To have short-term active and reactive power exchange ability during disturbance, power electronics-based energy capacitor system (ECS) has also been proposed recently. The two most promising short-term energy storage devices suitable for wind energy applications are flywheels and supercapacitors.
Thus, a supercapacitor-based grid fault ride-through system solving the aforementioned problems is desired.